A. Field of the Invention
The present invention relates to a semiconductor device.
B. Description of the Related Art
Generally, semiconductor devices are classified into a horizontal-type element in which electrodes are formed on one face of a semiconductor substrate and a vertical-type element in which electrodes are formed on both faces of a semiconductor substrate. In the vertical-type semiconductor device, a direction in which a drift current flows in the on-state and a direction in which a depletion layer grows in accordance with a reverse bias voltage in the off-state are the same. In a general planar-type n channel vertical-type MOSFET (insulated gate-type electric field effect transistor), a high-resistance n− drift layer part serves as an area that causes a drift current to flow in the vertical direction in the on-state. Accordingly, by shortening the current path in the n− drift layer, the drift resistance is lowered, and there is an advantage of lowering the substantial on-resistance of the MOSFET.
On the other hand, the high-resistance n− drift layer part is depleted so as to increase the breakdown voltage in the off-state. Accordingly, when the n− drift layer is thin, a spread width of a depletion layer between the drain and the base that advances from a p-n junction between a p-base area and the n− drift layer decreases, and the threshold electric field intensity of silicon is reached quickly, and the breakdown voltage decreases. In contrast to this, in a semiconductor device having a high breakdown voltage, since the n− drift layer is thick, the on-resistance is high, whereby the loss increases. As above, there is a trade-off relation between the on-resistance and the breakdown voltage.
It is known that such a trade-off relation similarly is formed in semiconductor devices such as an insulated gate bipolar transistor (IGBT), a bipolar transistor, and a diode. In addition, such a trade-off relation is common to horizontal-type semiconductor devices in which a direction in which a drift current flows in the on-state and a direction in which the depletion layer grows in accordance with a reverse bias in the off-state are different from each other.
As a solution for a problem according to the above-described tradeoff relation, a superjunction semiconductor device is known in which a drift layer is formed to have a parallel p-n structure having a configuration acquired by repeatedly bonding in an alternate manner an n-type area and a p-type area, of which the impurity densities are increased. In the semiconductor device having such a structure, by configuring total amounts of impurities of the n-type area and the p-type area of the parallel p-n structure to be the same as a whole, low on-resistance can be acquired while the breakdown voltage is maintained. Accordingly, in order to maintain the breakdown voltage of the semiconductor device having such a structure, it is necessary to control the total amounts of impurities of the n-type area and the p-type area of the parallel p-n structure with high accuracy.
In order to realize a high breakdown voltage of a semiconductor device, an element peripheral edge part structure is necessary. In a case where there is no element terminal structure, the electric field is high at the terminal of the drift layer so as to decrease the breakdown voltage, whereby it is difficult to realize a high breakdown voltage. As a structure for solving such a problem, it has been proposed to arrange a parallel p-n structure having a pitch smaller than that of the parallel p-n structure of an element activating part in a surface-side area of the outer circumference of the parallel p-n structure of the element activating part (see, for example, JP 2003-224273 A). According to the proposal, the surface electric field near the element activating part is alleviated, so that a high breakdown voltage is maintained.
In addition, even when a high breakdown voltage is maintained, in a semiconductor device having no electric charge resistance, the breakdown voltage decreases with the passage of time and accordingly it is difficult to assure the reliability of the breakdown voltage. As a structure for solving such a problem, a structure has been proposed in which an n− surface area is arranged in a surface-side area of a parallel p-n structure of an element peripheral edge part, and a field plate electrode that is electrically connected to a p-type guard ring area disposed inside the n− surface area is arranged on the surface of the n− surface area (see, for example, WO 2011/013379 A). According to such a proposal, a decrease in the breakdown voltage according to positive electric charge and negative electric charge can be suppressed.
As a semiconductor device that includes a p-type guard ring area and a field plate electrode, a semiconductor device having a configuration has been proposed in which a forward/reverse breakdown voltage structure unit includes first field limiting rings (FLRs), which are deep, disposed on the inner peripheral side, and second FLRs, which are shallow, disposed on the outer peripheral side, and insulating films that cover surfaces between a plurality of the first and second FLRs on the surface layer, and a conductive field plate that is in contact with the surfaces of a plurality of the FLRs overhang the surfaces of the insulating films disposed between the plurality of FLRs (see, for example, JP 2009-187994 A). In addition, as another device, a semiconductor device has been proposed in which, within the area of the upper face of the semiconductor unit, which has one conductivity type, a protection ring having the other conductivity type is disposed, and the protection ring is connected to a field plate (see, for example, JP 2000-101082 A and U.S. Pat. No. 6,274,904).
However, in a semiconductor device having low electric charge resistance, even when an initial breakdown voltage can be secured, the breakdown voltage decreases with the passage of time, and accordingly, there is a problem in that it is difficult to assure the reliability of the breakdown voltage. For example, in the semiconductor device disclosed in JP 2003-224273 A, when positive electric charge is present on an oxide film between the field plate electrode and a channel stopper electrode, it is difficult for the depletion layer to spread and therefore the electric field at the end of the field plate becomes high, such that there is a problem in that the breakdown voltage decreases. In the semiconductor devices disclosed in JP 2009-187994 A, JP 2000-101082 A, and U.S. Pat. No. 6,274,904, a parallel p-n layer is not disposed in the element peripheral edge part, and accordingly, there is a problem in that it is difficult to achieve a high breakdown voltage.
In addition, for example, in the semiconductor device disclosed in WO 2011/013379 A there are the following problems. FIG. 23 is a cross-sectional view that illustrates the configuration of a conventional semiconductor device. FIG. 23 is the superjunction semiconductor device that is disclosed in WO 2011/013379 A. As illustrated in FIG. 23, the conventional semiconductor device has element activating part 1 in which the surface structure of the element is disposed on a first principal face side, and element peripheral edge part 130 that surrounds element activating part 1 is disposed on the outer side of element activating part 1. On the first principal face side of element peripheral edge part 130, n− surface area 119 is disposed. On the first principal face side of n− surface area 119, three p-type guard ring areas 120a, 120b, and 120c are disposed so as to be separate from each other. The impurity density of p-type guard ring areas 120a, 120b, and 120c is higher than the impurity density of n− surface area 119.
In such a conventional semiconductor device, by arranging n− surface area 119 and p-type guard ring areas 120a, 120b, 120c, a high electric field near the outer circumference of element activating part 1 due to the presence of positive electric charge (positive ions) on oxide film 121 between field plate electrode 122 and channel stopper electrode 123 is alleviated. Accordingly, variations in the breakdown voltage due to the positive electric charge can be suppressed. However, in a case where there is positive electric charge of +1.0×1012 cm−2 or more between field plate electrode 122 and channel stopper electrode 123, even when n− surface area 119 is disposed on the first principal face side, it is difficult for the depletion layer to spread. Accordingly, the electric field becomes high at the end of field plate electrode 122, and therefore, the breakdown voltage decreases.
On the other hand, in a case where negative electric charge (negative ions) is present between field plate electrode 122 and channel stopper electrode 123, the depletion layer is prevented from reaching through the terminal of element peripheral edge part 130 due to field plate electrode 122 connected to p-type guard ring area 120c that is located on the outermost side. Accordingly, a decrease in the breakdown voltage due to negative electric charge can be suppressed. However, in a case where there is negative electric charge of −1.0×1012 cm−2 or less between field plate electrode 122 and channel stopper electrode 123, it is easy for the depletion layer to spread up to the end of channel stopper electrode 123 due to the arrangement of n− surface area 119 on the first principal face side. Therefore, the electric field becomes high at the end of channel stopper electrode 123, and therefore the breakdown voltage decreases.
As above, in the semiconductor device disclosed in WO 2011/013379 A, although the electric charge resistance of the breakdown voltage is considered, only electric charge resistance of a case where the amount Qss of the surface electric charge is −1.0×1012 cm−2 or more and +1.0×1012 cm−2 or less is secured. Accordingly, for a mold resin having a high impurity ion density, the electric charge resistance of the breakdown voltage is not sufficiently considered, and therefore there is concern that the breakdown voltage may decrease. In order to provide a superjunction semiconductor device having high reliability by avoiding a decrease in the breakdown voltage, it is necessary additionally to improve the electric charge resistance.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.